Identifying the factors that guide the development of organized connections in the cerebral cortex is crucial for preventing and treating neurological disorders resulting from abnormal development of neuronal circuits. This project stems from the realization that our knowledge of the development of cerebral circuits lags behind that acquired in living animals through the use of modern time-lapse imaging techniques. For instance, in Xenopus tadpoles, these techniques revealed highly dynamic phenomena involving the addition and retraction of small axon branches. These rapid phenomena may be critical for the formation of organized neural projections. In contrast, virtually all we know about the development of organized cortical circuits in mammals comes from studies using fixed tissue. The main goal of this project is to introduce the use of high resolution time-lapse imaging methods to the study of developing cortical projections in mammals. We will use acute slices of rat visual cortex to analyze the behavior of anterogradely labeled axons growing from area 17 to ipsilateral area 18. We chose this in vitro approach because at present there is no better alternative for observing the development of organized cortical projections in living mammalian tissue. Our preliminary time-lapse observations revealed a surprising level of activity in the form of branch additions and retractions that could not have been predicted using fixed tissue. We propose three experiments to examine this dynamic axon branch behavior at specific sites associated with critical events during the formation of organized cortical projections. If successful, our model will focus attention on the role that dynamic remodeling phenomena have in the development of cortico-cortical projections in mammals. Moreover, by allowing experimental manipulations that are not presently possible in living mammals, we expect that our model will advance our understanding of the cellular and molecular mechanisms underlying the development of cortical projection maps. [unreadable] [unreadable]